1 . Field of the Invention
The present invention relates to a display device and a touch sensing method thereof, and more particularly, to a display device with a built-in touch sensor and a touch sensing method thereof.
2 . Discussion of the Related Art
With the advancement of various portable electronic devices such as mobile terminals and notebook computers, the demand for flat panel display devices applied to the portable electronic devices is increasing.
Liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission display (FED) devices, light emitting diode (LED) display devices, and organic light emitting diode (OLED) display devices have been developed as flat panel display devices.
In such flat panel display devices, LCD devices are easily manufactured by advanced manufacturing technology and have drivability of drivers, low power consumption, high image quality, and a large screen, and thus, the application fields of the LCD devices are being expanded. LCD devices, including a built-in touch screen that enables a user to directly input information to a screen with a finger or a pen, are attracting much attention.
In applying a touch screen to LCD devices, a separately prepared touch panel is conventionally disposed on a liquid crystal panel, but, the LCD devices are recently developed in a type where the touch screen is built in a liquid crystal panel for slimming.
Especially, LCD devices using the existing elements, such as common electrodes formed in a lower substrate, as touch sensing electrodes are called in-cell touch LCD devices.
FIG. 1 is a diagram illustrating a structure of a touch screen panel in a related art in-cell touch type LCD device.
Referring to FIG. 1, the in-cell touch type LCD device uses a plurality of common electrodes, formed in a lower substrate (TFT array substrate), for display, and moreover uses the common electrodes as touch electrodes. In this case, the common electrodes are divided in units of pixels to configure a plurality of touch sensing blocks.
Specifically, among a plurality of pixels provided in the LCD device, a certain number of pixels (for example, 64×64 pixels) configures one touch sensing block. A plurality of touch sensing blocks are connected to each other in an X-axis direction to configure a plurality of touch driving electrodes 10 (TX), and a plurality of touch sensing electrodes 20 (RX) are arranged in a stripe type in a Y-axis direction. The touch driving electrodes 10 may be arranged in the X-axis direction identically to a direction of a gate line, and the touch sensing electrodes 20 may be arranged in the Y-axis direction identically to a direction of a data line.
In this case, a touch driving signal is applied to the touch driving electrodes 10 (TX), and the touch sensing electrodes 20 (RX) sense a capacitance change.
In in-cell touch LCD devices, display and touch sensing are temporally divided and driven due to a structural characteristic in which a plurality of pixels for display and a touch screen for touch detection are provided together.
During a touch sensing period (non-display period), when a capacitance is changed in a touch sensing block touched by a user's finger, a touch sensing electrode senses the changed capacitance, thereby determining whether there is the user's touch and a touched position.
In the related art, however, when touch sensors are disposed outside a liquid crystal panel, a display function and a touch sensing function are separately performed, and thus are not affected by each other. On the other hand, when the touch sensors are built in the liquid crystal panel, the display function and the touch sensing function are affected by each other. That is, touch sensing driving can affect display driving, or, the display driving can affect the touch sensing driving. This will now be described in more detail with reference to FIG. 2.
FIG. 2 is a view showing a capacitance change in a pixel displaying black and a capacitance change in a pixel displaying white, in the related art in-cell touch type LCD device.
As seen in FIG. 2, a pixel “Black” displaying black and a pixel “White” displaying white differ in alignment of liquid crystal, and thus, a difference between capacitances of a liquid crystal layer occurs.
The capacitance difference between pixels is maintained in the pixels even after a touch sensing period is started, and affects touch sensitivity. The capacitance difference functioning like offset to act as noise to touch sensing is called display touch crosstalk.
The accuracy and stability of a touch being reduced by the display touch crosstalk causes a bigger problem to a differential capacitive touch type in which a touch is sensed with a capacitance difference between adjacent touch sensing electrodes. This will be described in detail with reference to FIG. 3.
FIG. 3 is a diagram for describing a differential capacitive touch type in the related art in-cell touch type LCD device.
As seen in FIG. 3, in the differential capacitive touch type, the LCD device amplifies a capacitance difference (Cm(1)-Cm(2)) between adjacent touch sensing electrodes to determine whether there is a touch.
However, for example, when a pixel corresponding to one of adjacent touch sensing electrodes displays black and a pixel corresponding to the other displays white, a capacitance difference of a liquid crystal layer in the pixels displaying black and white acts like offset to a capacitance difference based on a touched position, causing display touch crosstalk. This will be described in detail with reference to FIG. 4.
FIG. 4 is a diagram showing a touch raw data difference between a touch sensing block displaying black and a touch sensing block displaying white in the related art in-cell touch type LCD device.
As seen in FIG. 4, the touch raw data difference between the touch sensing block displaying black and the touch sensing block displaying white is an average of 150 degrees. That is, even when there is no user's touch, a touch raw data difference is caused by a difference of display information displayed by a pixel corresponding to a corresponding touch sensing block.
For this reason, when a touch threshold value for detecting whether there is a user's touch is 100, even though there is no user's actual touch, a touch can be sensed.